1. Technical Field
The invention concerns a method for the production of long metal tubes with small diameters, where a metal tape is drawn from an annular coil, is gradually formed into a tube with a longitudinal slot and the longitudinal slot is welded, and a device for carrying out the method.
2. Description of the Prior Art
A method of producing an optical fiber cable element from a metal tube is known from DE 44 34 133 A1, where a metal tape is continuously drawn from an annular coil, it is formed into a tube with a longitudinal slot and the longitudinal slot is welded. In this case, the metal tube is used as protection for optical fibers which are inserted into the still open tube containing the longitudinal slot during the production of the metal tube. These metal tubes have a very small diameter, e.g. 3.5 mm and a wall thickness of 0.2 mm.
It is required or useful to be able to produce individual lengths of 6000 meters and longer, especially if such metal tubes with inserted optical fibers will be used in submarine or in overhead cables.
Since the annular coils which are delivered by the steel mills with the cited dimensions, i.e. a tape width of about 15 mm and a wall thickness of about 0.2 mm, can have a maximum tape length of 2000 to 2500 meters for reasons of annular coil stability and transportation. To obtain a 6000 meter length, the internally and externally located tape ends of the annular coils must be connected in series with so-called transverse welded seams.
Since the seam area must be stationary during the transverse seam welding, various possibilities have been developed in order not to interrupt the continuous tube production process.
One possibility is to rewind the metal tape before the tube production, i.e. the individual annular coils are rewound onto a larger supply coil, where the ends and the beginnings of two consecutive annular coils can be welded to each other while the winding process is interrupted. In this way, almost endless tape lengths can be produced with correspondingly larger supply coils.
Problems occur when the relation of the outer annular coil diameter to the tape width exceeds a certain size, since the stability of the annular coil is then lost and the individual windings begin to shift. Beyond that damage to the edges of the metal tape and also to the tape surface cannot be avoided in some metals because of cold-welding. Different tape expansions can also take place during the rewinding. Furthermore the rewinding process is very time-consuming and therefore costly.
A further possibility of producing tapes of long lengths includes introducing the metal tape drawn from the annular coil into a so-called storage device. When the end of the annular coil is reached, the end is held and welded to the beginning of a following annular coil. During the welding process, the tape material is removed from the storage device for continuous production (U.S. Pat. No. 5,191,911).
A device for producing longitudinally welded tubes from a metal tape is known from DE 26 04 324 B2, whereby several annular coils are tightly wound on a winding collar and the winding collar is arranged on a rotatable plate, while the longitudinal axis of the winding collar extends in the vertical direction. The metal tape is drawn first from the uppermost annular coil and is routed to the tube-shaping/welding station via a tape deflector and a tape storage device. When the first annular coil is used up, the beginning of the following annular coil is threaded into the tape deflector and routed to the transverse seam welding device, where it is welded to the tightly held end of the preceding annular coil. During these operations, the metal tape is routed from the tape storage device to the tube-shaping/welding station.
Working with a tape storage device proved to be unsatisfactory for very thin and narrow tapes, particularly with metals which have a cold-welding tendency.